Chewy steak help

I bought a whole scotch fillet (2kg) and have been cooking with that. It’s from the same cow and I have cut the pieces the same size. I used to do the method of sear all sides, place in oven for 4 minutes, and then rest and I always had good steak. I have tried sous vide using both rare setting and medium rare (using the app guide for time and temperature) and a quick sear after. Both times the meat is the correct color and doneness all the way through but it’s chewy. This is the same meat used for cooking in the oven as the sous vide, from the same cow but the sous vide makes it tough and chewy. I’ve tried seasoning before sous vide or after and seems to make no difference.

Scotch fillet is from the fore quarter of the cow. You’ll notice a bit of interconnecting muscle through it. You haven’t said how long you cooked it for sous vide, but I’d suggest 2 to 3 hours minimum.

Sous vide cooking works in two stages. Stage one will get the meat to temperature equilibrium, ie) the core of it to the same temperature as the water. This takes approximately 30 mins per half inch of thickness. For a naturally tender (unworked) muscle like tenderloin, this would be all that’s required. The second stage (which happens at the same temperature) creates tenderness by allowing time for the collagen holding the muscle fibres together to break down. For a steak you won’t want a full breakdown like you would if you were trying to get a ‘pulled’ finish, but you might require an extra hour or two on top of the equilibrium time. So, for an inch thick steak you could cook anything from 1 hour (equilibrium) to 3 hours. A 1.5" steak you might cook from 90 mins to 3.5 hours. It is recommended for anything over 4 hours cooking time that you do not cook below 130F/54C.

As you say, seasoning before or after makes little or no difference.